There is a command question that sums up the 2-column txt file.

Python 3.8.5 (tags/v3.8.5:580fbb0, Jul 20 2020, 15:57:54) [MSC v.1924 64 bit (AMD64)] on win32
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>>> import pandas as pd

>>> s1 = """E  DOS   PDOC_x
1    5    1
2    2    1
3    3    1
4    3    2"""
>>> from io import StringIO
>>> s2 = """E  DOS   PDOC_x
1    6    2
2    4    1
3    1    0
4    1    1"""
>>> s3 = """E  DOS   PDOC_x
1    3    2
2    3    1
3    4    3
4    4    1"""


>>> df1 = pd.read_csv(StringIO(s1), delim_whitespace=True)
>>> df1
   E  DOS  PDOC_x
0  1    5       1
1  2    2       1
2  3    3       1
3  4    3       2
>>> df1.info()
<class 'pandas.core.frame.DataFrame'>
RangeIndex: 4 entries, 0 to 3
Data columns (total 3 columns):
 #   #   Column  Non-Null Count  Dtype
---  ------  --------------  -----
 0   E       4 non-null      int64
 1   DOS     4 non-null      int64
 2   PDOC_x  4 non-null      int64
dtypes: int64(3)
memory usage: 224.0 bytes
>>> df2 = pd.read_csv(StringIO(s2), delim_whitespace=True)
>>> df3 = pd.read_csv(StringIO(s3), delim_whitespace=True)
>>> df2
   E  DOS  PDOC_x
0  1    6       2
1  2    4       1
2  3    1       0
3  4    1       1
>>> df3
   E  DOS  PDOC_x
0  1    3       2
1  2    3       1
2  3    4       3
3  4    4       1
>>> df1["DOS"] + df2["DOS"] + df3["DOS"]
0    14
1     9
2     8
3     8
Name: DOS, dtype: int64
>>> df1 = df1.set_index("E")
>>> df2 = df2.set_index("E")
>>> df3 = df3.set_index("E")
>>> df1["DOS"] + df2["DOS"] + df3["DOS"]
E
1    14
2     9
3     8
4     8
Name: DOS, dtype: int64
>>> df4 = pd.DataFrame()
>>> df4["DOC"] = df1["DOS"] + df2["DOS"] + df3["DOS"]
>>> df4
   DOC
E     
1   14
2    9
3    8
4    8
>>> df4.reset_index()
   E  DOC
0  1   14
1  2    9
2  3    8
3  4    8
>>>